[Python Programming (5)] Function and code reuse

1 Definition and use of functions

The function can have parameters or no parameters, but you must keep the brackets:

• def <function name> (<parameter (0 or more)>):
  　　<function body>
  　　return <return value>

1.1 Function parameter passing

Optional parameter passing : When defining a function, you can specify default values ​​for certain parameters to form optional parameters

• def <function name> (<non-optional parameter>, <optional parameter>):
  　　<function body>
  　　return <return value>

>>> def fact(n, m = 1):
	s = 1
	for i in range(1, n + 1):
		s *= i
	return s//m

>>> fact(10)
3628800
>>> fact(10, 5)
725760

Variable parameter passing : A variable number of parameters can be designed when the function is defined, using * b to achieve

• def <function name> (<parameter>, * b ):
  　　<function body>
  　　return <return value>

>>> def fact(n, *b):
	s = 1
	for i in range(1, n + 1):
		s *= i
	for item in b:
		s *= item
	return s

>>> fact(5, 2)
240
>>> fact(5, 2, 2, 5)
2400

There are two ways to pass parameters : when the function is called, the parameters can be passed by location or name

>>> def fact(n, m=1):
	s = 1
	for i in range(1, n + 1):
		s *= i
	return s//m

>>> fact(10, 5)
725760
>>> fact(m = 5, n = 10)
725760

1.2 The return value of the function

The function can return 0 or more results

• return reserved word is used to pass the return value
• The function may have a return value, or may not have, return may or may not

>>> def fact(n, m = 1):
	s = 1
	for i in range(1, n + 1):
		s *= i
	return s//m, n, m

>>> fact(10, 5)
(725760, 10, 5)
>>> a, b, c = fact(10, 5)
>>> print(a, b, c)
725760 10 5

1.3 Local variables and global variables

• Local variables are placeholders inside the function, which may be the same as global variables but different
• After the function operation ends, the local variables are released
• You can use global reserved words to use global variables inside functions

Local variables are combined data types and not created, equivalent to global variables

1.4 Lambda function

• The lambda function is an anonymous function, that is, a function without a name
• Use lambda reserved word definition, the function name is to return the result
• Lambda functions are used to define simple functions that can be represented on one line

example:

>>> f = lambda x, y : x + y
>>> f(10, 15)
25
>>> f = lambda : "lambda函数"
>>> print(f())
lambda函数

2 Example: Seven-segment digital tube drawing

• Step 1: Draw the digital tube corresponding to a single number
• Step 2: Obtain a string of numbers and draw the corresponding digital tube
• Step 3: Obtain the current system time and draw the corresponding digital tube

import turtle, time
def drawGap():              #绘制数码管间隔
    turtle.penup()
    turtle.fd(5)
    
def drawLine(draw):         #绘制单段数码管
    drawGap()
    turtle.pendown() if draw else turtle.penup()
    turtle.fd(40)
    drawGap()
    turtle.right(90)

def drawDight(dight):       #根据数字绘制七段数码管
    drawLine(True) if dight in [2,3,4,5,6,8,9] else drawLine(False)
    drawLine(True) if dight in [0,1,3,4,5,6,7,8,9] else drawLine(False)
    drawLine(True) if dight in [0,2,3,5,6,8,9] else drawLine(False)
    drawLine(True) if dight in [0,2,6,8] else drawLine(False)
    turtle.left(90)
    drawLine(True) if dight in [0,4,5,6,8,9] else drawLine(False)
    drawLine(True) if dight in [0,2,3,5,6,7,8,9] else drawLine(False)
    drawLine(True) if dight in [0,1,2,3,4,7,8,9] else drawLine(False)
    turtle.left(180)
    turtle.penup()
    turtle.fd(20)
def drawDate(data):         #data为日期，格式为 '%Y-%m=%d+'
    turtle.pencolor("red")
    for i in data:
        if i == '-':
            turtle.write("年", font = ("Arial", 18, "normal"))
            turtle.pencolor("green")
            turtle.fd(40)
        elif i == '=':
            turtle.write("月", font = ("Arial", 18, "normal"))
            turtle.pencolor("green")
            turtle.fd(40)
        elif i == '+':
            turtle.write("日", font = ("Arial", 18, "normal"))
        else:
            drawDight(eval(i))  #通过eval()函数将数字变为整数

def main():
    turtle.setup(800, 350, 200, 200)
    turtle.penup()
    turtle.fd(-300)
    turtle.pensize(5)
    drawDate(time.strftime("%Y-%m=%d+", time.gmtime()))
    turtle.hideturtle()
    turtle.done()
    
main()

operation result:

3 Function recursion

Recursion requires recursive formulas and recursive end conditions

Example 1: String inversion

def rvs(s):
    if s == "":
        return s
    else:
        return rvs(s[1:]) + s[0]

Example 2: Fibonacci sequence

def f(n):
    if n == 1 or n == 2:
        return 1
    else:
        return f(n-1)+ f(n-2)

Example 3: The Hanoi Tower problem The

solution process is as follows:
if n = 1, then move this plate directly from column A to column C.
If n> 1, perform the following three steps:

1. Using column C as a buffer, move the (n-1) plates on column A to column B
2. Move the last plate on column A directly to column C.
3. Using column A as a buffer, move the (n-1) plates on column B to column C.

count = 0
def hanoi(n, src, dst, mid):
    global count
    if n == 1:
        print("{}:{}->{}".format(1, src, dst))
        count += 1
    else:
        hanoi(n-1, src, mid, dst)
        print("{}:{}->{}".format(n, src, dst))
        count += 1
        hanoi(n-1, mid, dst, src)
hanoi(3, "A", "C", "B")
print(count)

4 Use of PyInstaller library

Function: PyInstaller library can convert .py source code into executable file without source code

PyInstaller library installation: (cmd command line) pip install pyinstaller

Simple to use: (cmd command line) pyinstaller -F <file name.py>

Common parameters:

parameter	description
-h	View help
–clean	Clean up temporary files during packaging
-D, --onedir	Default value, generate dist folder
-F, --onefile	Only generate independent package files in the dist folder
-i <icon file name.ico>	Specify the icon file used by the packager

example: Use the icon curve.ico as the icon of the executable file

• pyinstaller –i curve.ico –F SevenDigitsDrawV2.py

5 Example: Koch snowflake package

import turtle
def koch(size, n):
    if n == 0:              # 0阶时是一条直线
        turtle.fd(size)
    else:                   # 多阶时，分别针对4段绘制n-1阶科赫曲线
        for angle in [0, 60, -120, 60]:
            turtle.left(angle)
            koch(size/3, n-1)
def main():
    turtle.setup(600, 600)
    turtle.penup()
    turtle.goto(-200, 100)
    turtle.pendown()
    turtle.pensize(2)
    level = 3
    koch(400, level)
    turtle.right(120)
    koch(400, level)
    turtle.right(120)
    koch(400, level)
    turtle.hideturtle()
main()

operation result:

6 Summary

Definition and use of functions

• Use the reserved word def to define functions, and lambda to define anonymous functions
• Optional parameters (assign initial values), variable parameters (* b), name transfer
• The reserved word return can return any number of results
• The reserved word global declares the use of global variables, some implicit rules